In the field of an industrial machine including a movable structure which is numerically controlled, the movable structure is positioned at a position determined using a predetermined machine zero point as a reference under numerical control. Further, when the movable structure is operated in accordance with an NC program, the operation start position is usually set to the machine zero point. Therefore, the movable structure needs to be returned to the machine zero point in preparation for the operation before the NC program is started.
As one of methods of returning a movable structure to a machine zero point, a method disclosed in Japanese Unexamined Patent Application Publication No. 2000-137514 has conventionally been known. The zero return method disclosed in this document relates to an industrial robot controlled in accordance with an NC program, and in the method, movement of the robot to predetermined target positions is controlled by a subprogram and an operation program for returning the robot from each target position to the machine zero point is previously prepared, and, in the case where operation of the robot is interrupted before the robot reaches a certain target position, when restarting the robot, the robot is first moved to the target position, before reaching which the operation of the robot was interrupted, and then the operation program for returning the robot from the target position to the machine zero point is started to return the robot to the machine zero point.